The purpose of this research is to define better the vasoconstrictor action of thrombin that was reported recently. One phase of the study will determine whether alphathrombin and several other forms of thrombin (gamma and PMSF-treated) produce contractile responses in isolated arterial segments. Another phase will be a comparative doseresponse study with basilar, coronary, mesenteric and femoral arteries to reveal whether there are differences in sensitivity among vessels to the constrictor effect of thrombin. The reactivity of pial vessels to thrombin will be determined to reveal whether resistance arteries are influenced by this protein. Once the agonistic nature of thrombin is established, the blocking action of hirudin, which should specifically inhibit thrombin contractions (as opposed to those produced by serotonin and prostaglandin F(2 alpha)), will be quantified. Hirudin would serve as a specific tool for further experimentation. The ability of hirudin to block the vasoconstriction induced by whole blood on isolated arterial segments will be assessed. Hirudin also will be used to help elucidate the mechanism by which thrombin causes vasoconstriction. It is possible that the synthesis of prostaglandins by the arterial wall is responsible for the contractile effect of thrombin. Hence, hirudin should block both the contractions and the synthesis associated with vasoconstriction. Similarly, known inhibitors of prostaglandin synthesis (e.g., meclofenamate) should also block both of these effects of thrombin. Lastly, it is proposed that hirudin be used in an experimental model of subarachnoid hemorrhage. In these experiments hirudin will be added to blood prior to the injection of blood into the cisterna magna to degermine whether it prevents the cerebral vasospasm normally produced by this procedure. In another experiment, hirudin will be given as a subarachnoid wash to dogs 24-48 hours after cerebrovasospasm was produced by intrathecal blood on the assumption that thrombin is responsible for the prolonged spasm observed. The results will be compared with saline treated animals. The results of these studies may indicate that thrombin plays an important physiological role in hemostasis as a vasoconstrictor and a role in the pathogenesis of cerebral vasospasm that is associated with subarachnoid hemorrhage in humans.